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Relation of Crank and Eccentric.

637

and x as INSIDE OR EXHAUST LAP, forming an additional width
to the valve face, in line with valve spindle, on the steam or exhaust
edges of the valve respectively, for the purpose of giving early cut-off
to steam or exhaust. By adding steam lap the width of opening
is decreased, which is, however, compensated by giving increased
travel to the valve. Inside lap is rarely necessary, the alterations
in valve position caused by introducing stearn lap usually giving a
sufficiently early cut-off to exhaust (compression point). Various
interesting points are raised by altering the proportions of the
slide valve, which will be fully investigated later. (See p. 772.)

Relation of Crank and Eccentric.—The commonest
valve gear is the eccentric and rod. The eccentric is merely a
convenient form of crank whose pin is so enlarged as to envelop
the shaft: it follows that the eccentricity or length of eccentric
crank must be measured from centre of eccentric sheave to
centre of shaft. This amount we shall sometimes call the throw.
While, then, the piston moves the crank, the latter in turn moves
the eccentric, and so automatically, by the slide valve, adjusts the
supply of stearn.

( Without lap.} A normal valve must of necessity be at half
stroke when the piston is at the end of its stroke—that is, when
,the crank is at a dead centre^ for then the valve should be just
opening to steam. The eccentric crank must therefore be placed
at 90° to the engine crank. Further, the direction of rotation
will be determined by the position, right or left of it, of the
eccentric. The eccentric will always lead the crank or travel
before it; for, if we endeavour to turn oppositely, "we shall only
close the steam port at the very time itf-should be opening, and so
block the supply. Therefore, in a normal valve, the eccentric must
lead the crank by 90°.

( With lap.) Let us next consider a valve having lap. Re-
ferring, again, to Fig, 634, the thin outline shews a valve with
lap, placed at mid-stroke. It then covers the steam port plus the
lap. The crank being on dead-centre F, it follows that, in order
to admit steam by port R, valve must be moved bodily to the
right, and the eccentric lead the crank by 90° + /#/, as at HX. A
little consideration will shew that strictly similar conditions obtain
with the crank on the dead-centre z.